1. Field of the Invention
The present invention relates generally to reproduction of an optically recorded signal, and particularly concerned with the reproduction of an optically recorded signal of high frequency.
2. Description of the Related Art
Reproduction of a signal optically recorded on a surface of an optical disk with a thin recording film is accomplished by irradiating a track by a laser beam spot. For recording and reproducing signals of high density the system is as a means for a large capacity and high density data storing system. The recording density, however, is limited by the wavelength of laser used. FIG. 1 schematically shows the surface of an optical disk having U-shaped grooves 1 whereon signals are recorded. In the optical recording disk of FIG. 1 having the U-shaped grooves, in order to sufficiently minimize cross-talk from the neighboring grooves, the pitch between the parallel neighboring grooves (track pitch) has to be more than twice the wavelength of the laser light.
FIG. 2 schematically shows surface of an optical disk having V-shaped grooves 2 on which signals are to be recorded. In the optical recording disk of FIG. 2 having the V-shaped grooves, cross-talk from the neighboring grooves can be minimized even when the pitch between the bottoms of the V-shaped grooves are selected to be just twice the wavelength of the laser light. That is, the V-shaped groove type optical disk is capable of storing higher recording density by decreasing track pitches in comparison with those of U-shaped groove type recording disk (M. Nagashima; Applied Physics Letters, Vol. 42, P. 144, 1983 or U.S. Ser. No. 331,840 or EPC Application No. 81,110,606.1).
As the light source for recording and reproducing information on the optical disk, a semiconductor laser, which is capable of direct modulation and is small in size, is widely utilized. A high power semiconductor laser which can be used for recording the information and has a practically usable life time, for instance as shown in FIG. 3, has an elliptic light emitting facet 3 and sections of the emitted light beam 4 are also of elliptic shape. The wavelength of oscillated light is about 0.8 .mu.m and the power of emitted light is around 25 mW. In order to focus a circular spot on a surface of the optical disk, it is necessary to obtain a laser beam of substantially circular section by enlarging shorter diameter L of the section 4 of the laser beam.
FIG. 4 shows an optical system of the general configuration of an apparatus for recording and reproducing information signal on a disk. In the system, a light beam emitter part 57 has a semiconductor laser 5 to emit a coherent light, a collimate lens 6 for making the light from the semiconductor laser 5 a parallel light beam and a beam expander 7 which expands and reforms the collimated beam of the elliptic section to a beam of parallel light of substantially circular section. The laser beam X from the light beam emitter part 57 is then led to a polarized beam splitter 8, a quarter wave plate 9, and focussed by an objective lens 10 onto a recorded surface 11 of an optical disk. Light reflected from the recorded surface 11 is guided again through the objective lens 10, the quarter wave plate 9 and the polarized beam splitter 8, and then led through a convex lens 12, from which one part of the reflected light is further reflected by a mirror 13 to irradiate a photo-detector K for focussing control. The other part of the light from the convex lens 12 is led to another photo-detector M, and the signals detected by the photo-detectors M are used for reproducing the information and controlling tracking.
Shapes of the beam of the reflected light and the light receiving face of the photo-detector M for the system of U-shaped groove optical disk and for the system of V-shaped groove optical disk are shown in FIG. 5 and FIG. 6, respectively. Tracking of the light beam along the U-shaped grooves is made by adjusting the light intensity of the beam 16 received by light detection parts 14 and 15 to become equal each other, and by utilizing signals based on sum of the light intensity on the light detector parts 14 and 15, the signal is reproduced.
In case of the V-shaped groove type optical disk, the tracking control is made by slightly wobbling the V-shaped grooves to produce wobble containing output from the light detection parts 17 and 18 followed by extracting signals of the wobbling frequency therefrom (U.S. Ser. No. 525,412 and EPC Application No. 83,304,832.5).
In the optical signal recording system mentioned above, since beam forming by the beam expander 7 in FIG. 4 is necessary, light power transmission efficiency of the optical system is at largest only 40%; accordingly, even by using a high power semiconductor laser emitting light of 25 mW, actual light power received on the optical disk becomes only 10 mW. One example is described for a case of reproducing optically recorded signal by using a semiconductor laser which oscillates a light of the wavelength 0.83 .mu.m for a recording material of TeO.sub.x (T. Ohta, et al.; Journal of Applied Physics, Vol. 53, P. 8497, 1982 or M. Takenaga, et al.; Journal of Applied Physics, Vol. 54, P. 5376, 1983). In the example, for the optical disk of U-shaped grooves an objective lens of numerical aperture (NA) of 0.5 is used, and for the optical disk of V-shaped grooves an objective lens of numerical aperture (NA) of 0.6 is used in a manner that the laser beam is incident only to a central part of the lens corresponding to NA 0.5. The optical disk is rotated at 1800 rpm and the recording is made at a track having 75 mm radius (track linear velocity is 14 m/sec). By selecting recording power to be 8 mW and reproducing power to be 1 mW on the disk, a signal of 10 MHz (recording bit length is 0.7 .mu.m) is obtainable with C/N ratio of about 50 dB (with band width of 30 KHz). However, in order to handle high quality information (for instance, MUSE system of high quality TV signal proposed by NHK, Japan), a higher frequency signal must be recorded and reproduced.
The V-shaped groove type optical disk can enjoy about twice higher recording density in comparison with the U-shaped groove type by halving track pitch, but bit densities are the same in both systems and the quality of the information to be recorded has no difference. In order to elucidate behavior of reflected light from the optical disk, description is made hereafter how the bit density is limited by the laser light wavelength or NA of the objective lens.
As shown in FIG. 7, an orthogonal coordinate system (x-y) is provided on an entrance pupil of objective lens 10 for converging laser light on the disk surface, and the light wave distribution of the incident laser beam is represented by A(x,y). Another orthogonal coordinate system (.xi.-.eta.) is provided on the recording surface 11 of the optical disk. .xi. axis and .eta. axis are parallel with x axis and y axis, respectively, and the .xi. axis is set in radial direction of the optical disk, and .eta. axis is set in the tangential direction of the track of the optical disk. Complex reflectivity which gives effects on the reflected light wave of the laser spot is represented as R(.xi., .eta.). When it is provided that the recording face 11 of the optical disk has a periodic structure and its length of repetition in .xi. direction is p and that in .eta. axis direction is q, the complex reflectivity R(.xi., .eta.) can be represented by Fourier series expansion as follows: ##EQU1##
Further, on an exit pupil of the objective lens whereto the reflected light from the optical disk is incident, another orthogonal coordinate (u-v) is set. Axis u and axis v are made to agree with the x axis and the y axis. The reflected light wave distribution E(u, v) can be represented by the following equation: ##EQU2## The equation (2) represents the following reflected light from the optical disk consists of many diffracted lights E.sub.lm, and each of the diffracted lights has a similarity with the incident light A(x, y), and amplitude and phase of the light wave is determined by Fourier series coefficient R.sub.lm. Further, centers of respective diffracted light are disposed apart from each other by .lambda.lf/p in u axis direction and by .lambda.mf/q in v axis direction, wherein f is a focal length of the objective lens 10. These diffracted lights together, by superposing, namely interfering, form the reflected light wave E(u, v). Light intensity I(u, v) is the square of the absolute value of the light wave amplitude, and the reflected light intensity distribution I(u, v) is given as follows: EQU I(u,v)=E(u,v).sup.2 ( 3).
The recording list density is taken into consideration only along the track direction, and therefore the complex reflectivity distribution R(.xi., .eta.) can be considered in .eta. direction only in one dimensional way. Therefore, for simplicity, it is assumed that the distribution of the complex reflectivity distribution is uniform in .xi. direction. Accordingly, the equations (1) and (2) are represented in the following equations (4) and (5), respectively: ##EQU3##
Next, the case of FIG. 8 wherein recorded parts of reflectivity r.sub.2 each having length of 1/2q and unrecorded parts of reflectivity r.sub.1 each having length of 1/2q are disposed alternately with repetition length (i.e. spatial period) q. In this case, the complex reflectivity distribution R(.eta.) is represented as follows: ##EQU4##
Fourier series coefficients R.sub.m for the equation (6) is given as follows: ##EQU5## wherein the function Sinc(x) represents sin (x)/x. In case the optical disk moves by a distant of Vt (V is speed, t is time) in +.eta. direction, the complex reflectivity distribution, taking account of time change of FIG. 9, is represented as follows: ##EQU6##
From the equations (4) and (8), the time-changing Fouries series coefficients are given as follows: ##EQU7##
The conventional reproducing methods of FIG. 5 and FIG. 6 only receive reflected light intensities of specified regions. Such conventional method has a drawback that its bit density is not sufficient due to the below-mentioned reason.
The relation between the bit density and the reproduced signal is elucidated in detail in the following.
Firstly, the elucidation is made on the reproducing system for the optical disk having U-shaped grooves shown in FIG. 1.
Presumption is made that diameters of entrance pupil and exit pupil of the objective lens 10 of FIG. 7 are equal to the focal length f (namely NA=0.5), and that the incident light wholly enters the objective lens 10. Apart from FIG. 5 wherein the reflected light beam is semi-circular shape, here the section of the reflected light beam is presumed circular. FIG. 10 shows the diffracted lights E.sub.-1 (having center at 0), E.sub.+1 (having center at 0.sub.+1) and E.sub.-1 (having center at 0.sub.-1). Consideration is made by limiting only to the case wherein repetition length q of FIG. 8 and FIG. 9 are smaller than 2.lambda. (.lambda. is laser light wavelength). In the case of FIG. 10, the distance between the center 0 and center 0.sub.+1 and another distance between center 0 and center 0.sub.-1 are both larger than 1/2f. Each diffracted light has similarity with the incident light, and therefore, radii of respective diffracted lights are 1/2f. The circle E.sub.0 of FIG. 10 has the same size as the exit pupil, and within a region B therein the diffraction lights E.sub.0 and E.sub.+1 interfere with each other; within a region C the diffracted lights E.sub.0 and E.sub.-1 interfere with each other. Furthermore, within a region A, only the diffracted light E.sub.0 exists. For the simplicity, let us presume that incident light wave distribution is uniform and is A.sub.0, the light waves E.sub.B and E.sub.C within the regions B and C are represented as follows from the equations (5), (7) and (9): ##EQU8## Time changing component I.sub.B (t) and I.sub.C (t) of light intensities of these light waves E.sub.B and E.sub.C, respectively, are given as follows, since in a TeO.sub.x recording medium the reflectivity r.sub.1 and r.sub.2 can be considered as real numbers: ##EQU9##
In the diagram of FIG. 10, the light in the region A has no components that change with time. The area of the regions B and C of FIG. 10 is given by ##EQU10## and signal S(t) corresponding to the sum of the regions B and C is given as follows: ##EQU11## wherein .DELTA.R=r.sub.2.sup.2 -r.sub.1.sup.2. FIG. 11 is a diagram showing change of the reproduced signal amplitude with respect to repetition length q.
Apart from the above-mentioned recording medium of TeO.sub.x, for general recording materials, the reflectivities r.sub.1 and r.sub.2 may be complex numbers, and even for such case the similar relation holds.
The reproduction of the signal is made by each other interference of the reflected diffracted light of 0th order E.sub.0 with reflected diffracted light of +1th order E.sub.+1 or -1th order E.sub.-1, and when the interferences are not made (q&lt;.lambda.; that is, the regions B and C in FIG. 10 disappear), the reproduced light is not time-changing, i.e., constant with respect to time. When a recording medium of TeO.sub.x is used and laser light of wavelength 0.83 .mu.m is used, a signal of recording bit length of 0.7 .mu.m (namely, q=1.4 .mu.m) as above-mentioned is sufficiently usable for reproduction with C/N ratio of 50 dB. At this time, the amplitude of the reproduced signal is calculated from the equations (12) and (13) to be 0.186 (.DELTA.R)I.sub.0, by providing the incident light intensity I.sub.0 as ##EQU12##
Next, elucidation is made on reproduction of V-shaped groove type optical disk shown in FIG. 2 in brief. For the objective lens 10 of FIG. 7, a lens wherein diameters of entrance pupil and exit pupil are 1.2 times of the focal length f (namely, NA=0.6) is used, and the incident light beam 20 has an elliptic cross-section as shown in FIG. 12, wherein shorter diameter of the ellipse in the radial direction of the disk (in x axis direction) is selected to be equal to the focal length f. In the reproduction of signal on the V-shaped groove type optical disk, it is necessary to effectively utilize diffracted lights shifted in the radial direction of the disk (directions of the axes x, .xi. and u) (as disclosed in U.S. Ser. No. 331,840 and EPC Application No. 81,110,606.1. Therefore the one dimensional consideration made with reference to FIG. 8, FIG. 9 and FIG. 10 is not sufficient, but it is necessary to make calculation by means of the functions having two suffixes l and m as shown in the equations (1) and (2). Accordingly, at the exit pupil surface the case of two dimensionally expanding diffracted light as in FIG. 13 is considered. But the fundamental principle of using the interference of the diffracted lights in reproducing the signal is the same as the aforementioned elucidation with respect to the U-shaped groove optical disk. In the case of this V-shaped groove optical disk, the signal reproduction is made by making interferences on the exit pupil of the objective lens in a region outside a vertical straight line g, in a manner that diffracted lights E.sub.00 or E.sub.10 having the center on the U axis interfere with upward shifted diffracted lights E.sub.01 or E.sub.11 centers of which has shifted in v axis direction, within the region of the former diffracted light (E.sub.00 or E.sub.10). In the elucidation for FIG. 13, for simplicity, diffracted lights of l=-1 and m=-1 are omitted. There is a problem that for bit densities higher than a certain degree, the interference is not obtainable, hence making the reproduction of the signal impossible and this problem arises in V-shaped groove type optical disks quite similarly to the aforementioned case of the U-shaped groove type optical disk.
In order to handle a higher frequency signal with the same limited bit density, rotation speed of the optical disk must be increased. That is, by raising the track linear velocity, the frequency limit of the signal reproduction becomes higher for the same recording bit length. However, there occurs another problem that the recording material has an inherent limit of sensitivity of recording, and therefore, when the track linear velocity is increased, a considerably higher power of laser becomes necessary in order to carry out the recording with such high track linear velocity. Since the semiconductor laser has limited power, as aforementioned, transmission efficiency of the optical system must be made higher in order to raise the laser power on the optical disk. Therefore, magnifying factor of the incident laser beam must be decreased. FIG. 14 shows the situation of the entrance pupil of the objective lens at that time, wherein the elliptic line 21 designating the incident beam is drawn to designate the part having 1/e light intensity of that of the light axis. However, the laser spot 22 focussed on the optical disk becomes an ellipse which is longer in the track direction (.eta. axis direction) as shown in FIG. 15, and hence the bit density of the recording becomes low. Therefore, the above-mentioned measure can not attain an expected result intended by increasing the track linear velocity thereby failing to reproduce the high frequency signal.
Next, the above-mentioned matter is elucidated from view point of the operated of diffraction lights impinging on the exit pupil of the objective lens 10.
In FIG. 14, the incident light beam of elliptic section has its shorter diameter which is 0.5 times the focal length f of the objective lens, and for the simplicity it is provided that it has a uniform light intensity distribution .vertline.A.sub.1 .vertline..sup.2 within the elliptic area 21. At this time, the optical light transmission efficiency becomes about 60%, and by presuming that the lasing power of the semiconductor laser to be 25 mW, a light power at maximum of 15 mW can be collected on the disk. Then, by using the TeO.sub.x as recording material, by rotating an optical disk of a diameter of 20 cm at a speed of 3600 rpm, recording can be sufficiently made. On the exit pupil of the objective lens 10, as aforementioned the diffracted light has the expansion of the similar figure to the incident light. By handling the matter in one dimensional manner, similar to that of FIG. 10, wherein recorded bit length is 0.7 .mu.m (q=1.4 .mu.m) and laser wavelength is 0.83 .mu.m, the situation of the diffracted light becomes as shown in FIG. 16. Here, the distance between the centers of neighboring diffraction lights are about 0.6 f, and the interference region disappears, and the reproduced signal loses any time changing factor. Therefore, the above considerations prove that at a track of radius of 75 mm with a rotation speed of 3600 rpm, the highest frequency reproduceable signal is only 10 MHz. That is, even by improving the transfer efficiency and by increasing the rotation speed, the bit density is lowered, and it becomes impossible to reproduce signals of high frequency.
Summarizing the above, in the conventional art, as the recorded bit length is shortened, the centers of the diffracted light on the exit pupil of the objective lens depart more and more as shown in FIG. 10, FIG. 13 or FIG. 16, resulting in failure of interference between the diffracted lights, and disabling reproduction of high frequency signal.